1. Field of the Invention
The present invention relates to a process for removing helium and other impurities from a mixture containing deuterium and tritium. More particularly the present invention relates to the purification of spent plasma from its impurities in such a way as to recover deuterium and tritium in gaseous form for injection continuously or batch-wise into a fusion reactor.
2. Prior Art
The prior art is generally concerned with the purification of plasma continuously whilst it is in a gaseous form throughout. The principal steps of these known processes comprise the separation of deuterium and tritium by cryoadsorption methods, the H.sub.2 isotopes-separation by distillation at low temperatures, e.g. 20.degree. to 25.degree. K., and the storage and injection of the plasma into the fusion reactor. The equipment needed for such processes is very expensive and extremely bulky. It is expected to occupy approximately 300 m.sup.3 of building space. In the case of an accident, safety rules require auxiliary installations for the clean-up of the volume of the building occupied by the equipment. These installations are very expensive and require several days of continuous operation in the case of a serious accident, for instance the release of all of the tritium stored. The cost of the overall plant would be about 12 million dollars (1978), not including the cost of the building itself.
The most representative report about the design of such a plant is the report LA-6855-P of J. L. Anderson and R. M. Sherman. The loop in that plant is designed to handle 500 moles per day of DT.
A similar feasibility study carried out by Snia-Techint of Rome has given a higher cost, even when the processing rate of DT is reduced to about one half. Another purification method is described and claimed in our United Kingdom Patent Application No. 7902259 which is similar to the present invention but has some inconveniences, such as the relatively high tritium inventory and the applicability of the process to a reduced type of fusion reactor.
The present invention seeks to provide a process for removing helium and other impurities from a mixture containing deuterium and tritium, more particularly a process for the reprocessing of spent plasma removed from a thermofusion reactor, of improved economics and safety.
The process of the present invention is applicable to any plasma formed by mixtures of deuterium and tritium contaminated by the helium produced in accordance with the following fusion reaction: EQU D+T=.sup.4 He+neutron
Other impurities will probably also be present in the plasma stream such as CO, CO.sub.2, N.sub.2, NO, NO.sub.2, N(D,T).sub.3, C(D,T).sub.4 and C.sub.2 (D,T).sub.6 with a total concentration of about 2 to 3%. Even protium is expected to be present at a concentration of about 1% together with some microtraces of .sup.3 He.
All these impurities originate from many factors, of which the major ones are material degassing phenomena, air infiltration (even if only in micro-quantities), and some neutron reactions.
The prior art is replete with disclosures relating to the treatment of deuterium and tritium mixtures containing the above-mentioned impurities which are expected to accumulate in a real fusion burn.
Two disclosure form the basis of the known processing designs for such a fuel cycle. In the first one it is assumed that recourse can be made to extremely costly and very advanced procedures such as selective impurities cryogenic separation and hydrogen-isotope cryogenic distillation. The overall dimensions of such process units and the other units related to the overall process require a large facility for their containment. Moreover, in order to comply with safety rules it is a compulsory requirement to have an emergency tritium clean-up system in case of an accident. This system depends notably on the atmospheric volume of the plant. For this reason, a large investment and high running costs are demanded in order to maintain the release of tritium into the atmosphere below the present levels which are now becoming more and more strict.
The second processing design stems from the discovery of a process which allows for the complete cycling of the fuel. This process comprises the removal of exhaust plasma and its impurities out from the fusion reactor, the purification of the hydrogen from helium and impurities, the oxidation of the hydrogen isotopes to their oxides, their distillation to remove the non-tritium oxide-containing water and a mixture of deuterium/tritium oxides, their electrolysis to D.sub.2 and T.sub.2, and lastly the final injection thereof into the toroidal chamber of the reactor after their molar compositions have been correctly adjusted. With respect to the former known process, this latter known process should reduce the containment problems, meet the safety requirements and constraints better, and reduce the volume of the overall plant, which in turn will reduce the cost of the system required for the smaller tritium clean-up emergency system.
On the other hand, the tritium inventory of this latter known process seems to be greater than that of the former known process, and also its application finds some difficulties when the plasma is heated by deuterium neutron beams because the water and tritium-contaminated deuterium stream require either a complementary purification system or the use of a larger column which of course increases the tritium inventory.